Prey identification by echolocating bats relies upon short term integration of the frequency modulations created by an insect's beating wings across echoes. Behavioral studies have shown that bats actively increase their sonar emission rate from low to high as they approach potential prey. Recent studies in the inferior colliculus (IC) have shown that this increase in rate results in a narrowing of amplitude response bandwidth; presumably increasing the amplitude resolution of the system. A fundamental question is how the stimulation rate (SR) affects the processing of other aspects of sound, including frequency. Therefore, two specific aims will be addressed. Aim 1 will test the influence of SR on the frequency response properties of IC units. We will compare single unit responses to sound stimuli at low and high SRs. Additionally, we will examine the minimum number of pulses necessary to generate rate-dependent changes in frequency selectivity. Aim 2 will study the influence of inhibition in the rate-dependent changes via iontophoretic application of GABAa and glycine receptor antagonists, thereby testing the hypothesis that an increased efficacy of inhibition at higher rates produces the rate-dependent sharpening of frequency selectivity.